Switches utilizing capacitive sensing user interfaces have been utilized in various electronic devices and electromechanical devices, such as cellular phones, audio/video players, and elevators. These switches may implement various functions, including simple on/off state functions. Without moving parts, switches utilizing capacitive sensing user interfaces typically have higher durability and better resistance to contaminants than other types of switches, such as dome-switches, mechanical-switches, etc.
A capacitive sensing user interface for implementing a switch may include a set of capacitive sensing elements, e.g., capacitive sensing buttons, keys, or switches, as discussed with reference to FIGS. 1A-B. FIG. 1A illustrates a schematic representation of a top view of a capacitive sensing button 100, hereinafter “button 100”; FIG. 1B illustrates a cross-sectional view of button 100.
Button 100 may include an anode/sensor 102 and a cathode/ground 104, both typically made of a conductive material, such as copper. Anode 102 may be coupled with a signal source (e.g., a voltage or a current source) and may receive charge from the signal source through a conductive trace 112 (shown in FIG. 1B). Cathode 104 may be disposed such that a gap 106 exists between anode 102 and cathode 104. Accordingly, anode 102 and cathode 104 may form a capacitive device.
Button 100 may also include a substrate 110 for supporting anode 102 and cathode 104. Substrate 110 may represent a flex circuit board and may be made of polyimide, such as Kapton® available from E. I. du Pont de Nemours and Company, www.dupont.com. Button 100 may also include a cover or lens 108 for providing mechanical, environmental, and/or electrostatic discharge (ESD) protection for button 100.
A human finger typically has higher conductivity than the air surrounding button 100 and in gap 106. Therefore, if a finger is in the vicinity of button 100, for example, touching lens 108, the capacitance between anode 102 and cathode 104 may increase. By appropriately detecting the increase in capacitance due to the presence of the human finger, a switch may thus be implemented by the capacitive sensing interface of FIGS. 1A and 1B.
Although only a single button 100 is discussed in connection with FIGS. 1A and 1B, it should be understood that a capacitive sensing user interface may include any number of capacitive sensing buttons, similar to button 100, for activating (and deactivating) a set of functions of an electronic or electromechanical device.
To further elaborate, FIG. 2A illustrates a schematic representation of an example state-determining circuitry of a representative capacitive sensing user interface system 200, hereinafter “user interface 200.” User interface 200 may include, for illustration, a capacitive sensing button 202, representing a single button of the capacitive sensing user interface. Another capacitor 234 is also shown, representing the capacitive effect experienced when the user's finger is in the vicinity of user interface 200.
User interface 200 may further include state-determining logic comprising a driver 212, a comparator 208, an AND gate 220, a timer 218 (or counter 218), and a processor/controller 216. Generally speaking, the state-determining logic operates in two phases: the precharge phase and the discharge phase. These two phases (i.e., precharge phase and discharge phase) are shown in FIG. 2B. The state-determining logic charges up the capacitor during the precharge phase and monitors during the discharge phase the rate at which the voltage on the capacitor's plate decays. From the rate of decay of the capacitor's plate voltage, the state-determining logic then determines whether a user's finger is present in (or absent from) the vicinity of the switch associated with that capacitor. The operation of the state-determining logic of FIG. 2A will now be discussed in connection with the signals of FIG. 2B, which are obtained at various nodes (e.g. node 222) of the state-determining circuitry illustrated in the example of FIG. 2A.
Generally speaking, driver 212 is coupled to receive a supply voltage VDD from a voltage or current source (not shown to simplify the illustration). At the start of the precharge phase, DIR signal 230 is set to high (as evidenced by the low-to-high transition 250 of the DIR signal in FIG. 2B). As long as the DIR signal 230 is high, driver 212 provides this supply voltage VDD to node 222 to charge up capacitor 202. With reference to FIG. 2B, the high state of DIR signal 230 is shown by reference number 292. With driver 212 supplying supply voltage VDD to node 222 during the precharging phase (characterized by the high DIR signal in FIG. 2B), the voltage at node 222 is shown rising over time to a voltage level 294 (i.e., some voltage level at or below VDD), reflecting the fact that capacitor 202 is charged during the time when DIR signal 230 is high. During this precharging period, the counter of counter circuit 218 is set to zero to prepare for counting during the subsequent discharge period.
At some point in time, a processor turns off DIR signal 230, as evidenced by the high-to-low transition edge 256 of DIR signal 230 in FIG. 2B. Turning off DIR signal 230 begins the discharge phase and activates counter 218 to begin counting as will be discussed later herein.
With DIR signal 230 turned off, driver 212 no longer supplies VDD to node 222 of FIG. 2A. Node 222, which is coupled to capacitor 202 and is charged up prior to DIR signal 230 being turned off, now begins to discharge through a resistor 232. This discharge causes the voltage level at node 222 to gradually decay (as seen by the downward slope 258 of signal V222 in FIG. 2B).
Since node 222 is input into comparator 208 to compare the voltage at node 222 (V222) against a threshold voltage VTH, the output of comparator 208 will be high during the time that the voltage at node 222 (V222) exceeds the threshold voltage VTH. On the other hand, the output of comparator 208 will be low when the voltage at node 222 (V222) decays to below the threshold voltage VTH. Generally speaking, the threshold voltage VTH for a given button may depend on factors such as, for example, the component sizes, the construction details, and the materials employed that are specific to that given button.
In the example of FIGS. 2A and 2B, comparator 208 is implemented to output the comparison result only during the discharge phase, resulting in the comparator output 224 signal shown in FIG. 2B. In this implementation, the output of comparator 208 goes high at edge 260 the start of the discharge phase (coincident with falling edge 256 of DIR signal 230) and stays high until the voltage at node 222 (V222) falls below the threshold voltage value VTH. When the voltage at node 222 (V222) crosses the threshold value VTH (shown by reference number 262 on the V222 signal of FIG. 2B), comparator output 224 goes low at edge 264.
With reference to the circuit diagram of FIG. 2A, note that the output 224 of comparator 208 is inputted into an AND gate 220. The operation of AND gate 220 is such that when both comparator output 224 and a periodic clock signal 236 are high, a high output is produced at AND output 238. This situation occurs during the high portions 270, 272, 274, 276, 278, and 280 of periodic clock signal 236 in FIG. 2B since comparator output 224 is also high during that time. The resultant signal for AND output 238 is shown in FIG. 2B, showing six pulses 280-290 to mirror the six clock cycles 270-280 of 236 during the time comparator output 224 is also high. Counter 218, which is activated by way of Enable signal 240 at the start of the discharge phase to begin counting, will count these six pulses 280-290 and derive from the period of 236 the decay time it took for the voltage at node 222 (V222) to decay from its charged value to threshold voltage VTH. If the period of 236 is about 10 ms, the six pulses will be about 60 ms, for example. Accordingly, the decay time sample is 60 ms.
An alternate implementation may have comparator 208 outputting a high value as soon as the voltage at node 222 (V222) exceeds threshold voltage VTH during the precharge phase. This implementation (which is not shown in FIG. 2B) may result in AND gate outputting pulses before the start of the discharge phase (i.e., as soon as the output of comparator 208 goes high). However, since counter 218 does not start counting until the start of the discharge phase, only six pulses 280-290 will be counted, and the result is the same as far as the value of the decay time.
Note that even though only one capacitor 202 is shown in FIG. 2A, multiple capacitors implementing multiple capacitive sensing switches may be provided. Using an appropriate multiplexing circuit and a selector signal (which may be controlled by processor 216), individual capacitors may be polled for their decay times.
So far, the capacitance effect of the user's finger being in the vicinity of the capacitor sensing interface (e.g., capacitor 202 of FIGS. 2A and 2B) has not been discussed. This capacitance effect is modeled in FIG. 2A by parallel capacitor 234 and is now discussed with reference to FIG. 2C hereinbelow.
FIG. 2C illustrates the use of a decay time sample to determine the on/off state of a given button. In the absence of the user's finger, curve section 252 may represent the decay of voltage V at node 222 (see FIG. 2A) after supply voltage VDD is removed from node 222. On the other hand, when a user's finger is present in the vicinity of the capacitive sensing interface, curve section 254 may represent the decay of voltage V at node 222 after supply voltage VDD is removed from node 222.
Since the presence of the user's finger tends to increase the capacitance of the button, it may take a longer time during the discharge phase for voltage V at node 222 to decay from its charged value (which is charged up during the precharge phase as discussed earlier) to threshold voltage value VTH. Accordingly, as illustrated in the example of FIG. 2C, a decay time tp associated with the presence of the finger is greater than a decay time ta associated with the absence of the finger. By comparing the decay time sample measured for a button with a threshold characteristic time tTH, it is possible to detect whether a user's finger is present near a capacitive sensing button or whether the user's finger is absent.
Generally speaking, as in the case with the threshold voltage VTH, the threshold characteristic time tTH may be predefined for each button. The threshold characteristic time tTH for a given button may depend on factors such as, for example, the component sizes, the construction details, and the materials employed that are specific to that given button. When a decay time sample is greater than tTH, the button is deemed to be activated by the user, and the function associated with the button is activated (e.g., the on state). When the decay time sample is less than tTH, the button is deemed to be not activated by the user. Accordingly, the function associated with that button is not activated (e.g., the off state).
It is generally known, however, that there may exist various ambient and/or transient noises in the operating environment of capacitive sensing user interfaces. For example, there may be noises generated by the radio frequency (RF) circuitry in a cellular phone or by the 60 Hz AC line voltage in a typical home or office/factory environment.
If a single decay time sample is employed for determining the on/off state of a button, random transient noises (such as noises from a nearby radio frequency (RF) circuit or 60 Hz line voltage) may unduly affect the accuracy of the on/off state determination. This is because the presence of transient noises may momentarily increase the capacitive value of a given button. If the decay time sample for that button happens to be measured while the button is affected by transient noises, the resultant on/off state determination may be erroneous. For example, it is possible that the on/off state-determining logic may misinterpret the increase in the decay time sample due to transient noises as “user's finger present” and may deem the switch to be activated by the user even though the user's finger is nowhere near the capacitive sensing button of that switch.
To ameliorate the error-inducing effects of transient noises, multiple decay time samples may be sampled consecutively and statistically processed (e.g., averaged) for a capacitive sensing button before the on/off state determination is undertaken. With reference to FIG. 2B, DIR signal 230 may be turned on and off multiple times for each capacitive sensing button to generate multiple decay time measurements (with each decay time measurement comprising a plurality of pulses) for that button. With reference to FIG. 2A, multiple decay time samples may be received from counter 218 and averaged by processor/controller 216 to determine the on/off state of the button.
Since it is unlikely that transient noises may last long enough to affect multiple decay time sample measurements, the error-inducing effect of transients on the state determination process may be reduced simply by taking multiple decay time samples for each button before determining the on/off state of for that button. On the other hand, since a human finger moves relatively slowly, the detection of a long decay time in multiple decay time measurements are likely to indicate the presence of a user's finger.
To elaborate, suppose six hypothetical consecutive decay time samples for a hypothetical capacitive sensing button are obtained for capacitor 202 of FIG. 2A. The values of the decay time samples may be 10 microseconds, 12 microseconds, 14 microseconds, 184 microseconds, 16 microseconds, and 17 microseconds. In this case, since the long decay time sample occurs only once, it is likely that the increase in decay time of 184 microseconds is due to a transient event. In this case, the processor may simply average out the anomalous sample or may ignore the anomalous sample altogether. On the other hand, if the decay time samples are 10 microseconds, 180 microseconds, 192 microseconds, 188 microseconds, 174 microseconds, 190 microseconds, the fact that five out of six decay time samples show a long decay time is probably indicative of the presence of a user's finger in the vicinity of the capacitive sensing button.
FIG. 3 illustrates an example prior art scheme for determining on/off states of capacitive sensing buttons utilizing multiple consecutive decay time samples for each button. In the example of FIG. 3, curve sections 302-304 may represent the voltage decay associated with button 202 in multiple decay sampling cycles. Curve sections 306-308 may represent voltage decay associated with a different button 204 in multiple decay sample cycles, and curve sections 310-312 may represent voltage decay associated with a different button 206 in multiple decay sampling cycles
Decay time samples 322-324 may be acquired consecutively and then averaged to determine the on/off state of button 202. The process may then continue in a round-robin fashion with button 204 and button 206. For example, after the on/off state of button 202 is determined, decay time samples 326-328 may be consecutively acquired and then averaged to determine the on/off state of button 204. After the on/off state of button 204 is determined, decay time samples 330-332 may be consecutively acquired and then averaged to determine the on/off state of button 206.
There are, however, disadvantages associated with the prior art method for determining on/off states of capacitive sensing buttons. As illustrated in the example of FIG. 3, the on/off state of button 206 may not be determined until after a substantial period of time, including the time needed to acquire multiple decay time samples for button 202 and button 204, has elapsed. Thus, in the example of FIG. 2, at least four decay time determining cycles (associated with decay time samples 322, 324, 326, and 328 for buttons 202 and 204) elapse before button 206 has its turn to have its on/off state determined. Although the example of FIG. 3 measures only two decay time samples for each button, if a greater number decay time samples per button is desired (e.g., 6 or 20 decay time samples for each button), button 206 may have to wait for a significant amount of time before its turn to have the on/off state of its button determined. In the electronic device application, if the delay in determining the on/off state for a button is longer than a certain period of time, such as 20 ms for example, that delay may be perceived by a user of the capacitive sensing user interface. As a result, the user may perceive the electronic device as “slow” or “unresponsive” and the user experience may not be satisfactory.
Further, if there exists a noise which substantially overlaps the time during which the multiple decay time samples for a given button are acquired, that noise may significantly affect the accuracy of decay time determination and may tend to cause false positives or false negatives (or incorrect state determination in general) for that given button. This is so even if multiple decay time samples are acquired and averaged before the on/off state determination is made. For example, if a quasi-periodic noise tends to concentrate between time tr and time tx of example FIG. 3, the quasi-periodic noise overlaps the multiple decay time samples acquired for button 202 and may cause false positives with respect to button 202 even if multiple decay time samples are obtained and averaged before the on/off state of button 202 is determined.